Halogenoalkanes Flashcards
Complete!
General formula of halogenoalkanes
CnH2n+1X (X = halogen)
What prefixes tell us what halogen is present?
fluoro-
chloro-
iodo-
What prefixes tell us how many atoms of each halogen are present?
di-
tri-
tetra-
How is compound named when containing different halogens?
Listed in alphabetical order, not in order of the number of carbon atoms to which they are bonded
Is the C–X bond polar?
Yes
C (delta +)
X (delta -)
What happens to polarity of bond going down the group?
Gets less polar
Are halogenoalkanes soluble in water?
No
C–X bonds aren’t polar enough to make them soluble in water
What are the main intermolecular forces in halogenoalkanes?
dipole-dipole attractions and van Der Waal forces
What uses can there be when halogenoalkanes mix with hydrocarbons?
Can be used as dry-cleaning fluids and to remove oily stains
The boiling point _______ with _______ chain length
The boiling point INCREASES with INCREASED chain length
The boiling point _______ going _______ the halogen group
The boiling point INCREASES going DOWN the halogen group
Why does boiling point increase with:
- Increased chain length
- Going down the halogen group
Increased van Der Waals forces because the larger molecules, the greater the number of electrons
Therefore, the larger the van Der Waals forces
What is the difference of boiling points between halogenoalkanes and alkanes (with similar chain lengths)?
Why?
Halogenoalkanes have HIGHER boiling points than alkanes with similar chain lengths
They have higher relative molecular masses and they are more polar
What are the two factors that determine how readily the C–X bond reacts?
Which is more important?
C–X bond polarity
C–X bond enthalpy (more important)
The carbon bonded to the halogen has a partial positive charge. It is electron _______
It is electron deficient
Definition of nucleophiles
Species that has a lone pair of electrons with which it can form a covalent bond by donating its electrons to an electron deficient carbon atom
(positive charge loving) - attracted to positively charges species
Is C–F or C–I most or least reactive?
Why?
C–F most reactive - most polar therefore more likely to be attacked by a nucleophile
C–I least reactive - least polar
The bonds get _______ going down the group
The bonds get WEAKER going down the group
Why is C–F a strong bond?
Fluorine is smallest atom
Shared electrons in C–F bond are strongly attracted the fluorine nucleus
Why does the C–X bond get weaker going down group 7?
Shared electrons in the C–X bond get further away from halogen nucleus
What charge does a nucleophile have?
In what form?
Negative
Either negatively charged ion or has an atom with delta - charge
What does a nucleophile contain?
What does this form?
Lone pair of electrons
Used to form a covalent bond
What are the common nucleophiles?
How should they be written?
Hydroxide ion (OH-) Ammonia (NH3) Cyanide ion (CN-)
(have to be shown with lone pair and -, except NH3, no -)
What can nucleophilic substitution be used to produce?
Alcohols and amines
What does the lower curly arrow show?
What does the halogen atom become?
The electron pair in the C–X moving to the halogen atom (X)
A halide ion (leaving group)
What is the leaving group in nucleophilic substitution?
The halide ion
What does the rate of substitution depend on?
Example:
The halogen
Fluoro-compounds unreactive due to strength of C–F bond
Going down the group, the rate of reaction _______ as the C–X bond strength _______
Going down the group, the rate of reaction INCREASES as the C–X bond strength DECREASES
Conditions for -OH nucleophile (aqueous sodium or potassium hydroxide)
- Warm the mixture (slow at room temp)
- Ethanol as a solvent (so halogenoalkanes and aqueous sodium or potassium hydroxide both mix - hydrolysis reaction)
Condition for -CN nucleophile (cyanide ions)
In ethanol
Conditions for NH3 nucleophile
- In ethanol
- Heat in sealed tube
- Carried out under pressure
What nucleophile (+ halogenoalkanes) produce alcohol?
OH- (H20)
What nucleophile (+ halogenoalkanes) produce nitrile?
CN- (KCN or NaCN)
What nucleophile (+ halogenoalkanes) produce amine?
NH3 (concentrated NH3)
Definition of elimination reaction?
A reaction in which a molecule loses atoms or groups of atoms to form a double C bond
Product of an elimination reaction
Alkene (hydrogen halide eliminated leaving double bond in its place)
What is OH- in an elimination reaction (under different conditions than in nucleophilic substitution)?
What does it do?
Acts as a base
Removed an H+ ion from halogenoalkane
What conditions needed for elimination reaction with OH- (sodium/potassium hydroxide)?
Sodium/potassium hydroxide dissolved in ethanol and mixed with halogenoalkane
Mixture heated
Would OH- at room temp, dissolved in water favour substitution or elimination?
Substitution
Would OH- at high temp, dissolved in ethanol favour substitution or elimination?
Elimination
Does elimination need ethanol or water as a solvent?
Ethanol
Does substitution need ethanol or water as a solvent?
Water
Does a high temperature encourage elimination or substitution?
Elimination
Does a higher concentration encourage elimination or substitution?
Elimination
Primary halogenoalkane - nucleophilic substitution or nucleophilic elimination?
Nucleophilic substitution
Tertiary halogenoalkane - nucleophilic substitution or nucleophilic elimination?
Nucleophilic elimination
What are chlorofluorocarbons (CFCs)?
Halogenoalkanes which only contain carbon, chlorine and fluorine - NO HYDROGEN
What were the short chain CFCs (gases) used for?
E.g. Refrigerants
What were the long chain CFCs used for?
Dry cleaning and de-greasing solvents
What happens when CFC gases end up in the atmosphere?
They decompose to give chlorine atoms
Chlorine atoms decompose ozone (O3) in the stratosphere, which caused hole in Earths ozone layer
What was the supposed benefits of CFCs?
Non toxic
Non flammability
What has been developed instead of CFCs?
HCFCs - contain hydrogen
Decompose more easily than CFCs due to C–H bonds and chlorine atoms released lower in the atmosphere where they do not contribute to destruction of ozone layer
…and also HFCs - no chlorine
Therefore no damage to ozone layer
